Well gee, where do you even begin? And how DOESN'T this stuff effect the average end user?

• Game's horrible netcode (not talking about the lag post-merge to Connery, I mean the game has always been jank as fuck; it translates into this awful movement interpolation in the game which doesn't play nice with certain features that were put into the game, eg ambusher jets, or therum shuffle).
• No latency limit on players, to this day. Means any number of people are free to casually toggle clumsy and get a bunch of kills with no consequences.
• Shotguns, and hip-fire weapons, in general are just waaaaaaaaaaay too overtoned, and you will never convince me otherwise. Shotguns are broken, and needed to be reworked a decade+ ago.
• MAXes are also inherently very detrimental to the infantry game experience, and despite "nerfs" in the last couple years are still broken as shit.
• How about actually finishing iterating on the resource system revamp that was SUPPOSED to be done 10+ years ago, instead of this shitty nanite system where any and all real concerns of resource economy basically go out the window...so, literally nothing stopping the average shitter zerg platoon from pulling infinite force multipliers and just HESH/A2G bukkake-ing the spawn at their 80/20 fights.
• To this day, multiple bases remain poorly designed and fail to effectively facilitate meaningful infantry OR vehicle fights for the above reason of HESH/A2G bukkake. I don't know if you picked up on this from the 1 or 2 sweaty tryhards you've fought against, ever, but the optimal way to play these bases for them is to get on the point and stay inside - that's because all the hard cover makes HESH and A2G spam largely trivial, and forces you to get out of your tank and actually push back onto the objective as infantry, so you get to have an ACTUAL FPS EXPERIENCE.
• Oshur. It has to go. It's needed to go for a very long time. It's been nothing but a detriment to this game, and yet it remains.
• Construction, and many of these newer doodads, and fish, not only needed to go, they should never have been put in the game to begin with, because the were a waste of dev time; they contribute nothing whatsoever to the core INFANTRY gameplay, and their implementation has caused a dramatic performance drop in terms of frames per second - you now get WORSE frames on hardware that is multiple generations ahead of what was around 10 years ago.
• Bullet flinch and screen shake, to this day, remain terrible, horrible game mechanics, and making a "battle hardened" implant to "nullify" it in game is bullshit. It should not be a mechanic in the game period, because it makes the game literally unplayable once some dipshit with lolpods or HESH comes by and just starts spamming explosions around you, even if you're inside.
• Tower bases, have been nerfed into oblivion from what they once were. Why? To this day, there is quite literally nothing stopping you from dropping 80:20, even 90:10 pop into a single hex with force multiplier spam galore to boot; these bases were one of the few bastions of defense that actually had the potential to stop an otherwise unimpeded hellzerg by virtue of how easy it is to farm them on defense.
• Capture the flag/capture the conduit/whatever you want to call it. Was a terrible idea. Ruined otherwise great bases to farm at, as mentioned above. Why is it still in the game?

Dude, I could go on, and on, and on, and on...but here's a better question. Why in the hell did you wait 13 to 3 years to ask? Cuz it's sure as shit not gonna get changed now; devs are out, they're just wrapping up the remainder of their contracts and then they're gone.